Table 1.--Format of gravity data [ft, feet; g/cc, grams/cubic-centimeter; mGal, milligal] ______________________________________________________________________ Column Format Description ______________________________________________________________________ 1-8 A8 Station name 10-11 F2.0 Latitude, in degrees 12-15 F4.2 Latitude, in minutes to 0.01 minute 17-19 F3.0 Longitude, in degrees 20-23 F4.2 Longitude, in minutes to 0.01 minute 24-29 F6.1 Elevation, in ft to 0.1 ft 30-36 F7.2 Observed gravity, in mGal to 0.01 mGal, without leading 9 37-40 A4 Four character accuracy code 41-46 F6.2 Free-air anomaly, in mGal to 0.01 mGal 47-52 F6.2 Simple Bouguer anomaly, in mGal to 0.01 mGal; or date. 53-57 F5.2 Inner-zone terrain correction, in mGal to 0.01 mGal 58-62 F5.2 Total terrain correction, in mGal to 0.01 mGal 63 A1 Terrain correction code denoting extent of inner-zone terrain correction and system, upper case denotes Hayford-Bowie system of zones, lower case denotes Hammer system of zones. Z, computer calculated from 0.0-0.59 km 64-69 F6.2 Complete Bouguer anomaly reduced for a density of 2.67 g/cc, in mGal to 0.01 mGal 70-75 F6.2 Isostatic gravity anomaly reduced for a density of 2.67 g/cc, in mGal to 0.01 mGal ______________________________________________________________________ Table 2.--Explanation of 4-digit accuracy code [alt, altimetry; blk, black; brn, brown; elev., elevation; NGS, National Geodetic Survey; NMD, National Mapping Division; PG, photogrammetry; USGS, U. S. Geological Survey; VABM, vertical angle bench mark] ____________________________________________________________________ General elevation and location code--First digit Code Explanation Code Explanation ____________________________________________________________________ A Altimetry, good control P On or near surveyed mark B On USGS or NGS level-line bench mark Q River gradient interpolation C Contour line interpolation R Lake elev. by leveling D Destroyed or not found reference mark S Sea level elev. E Near non-USGS level-line bench mark T Photogrammetry by USGS NMD F Map elev., blk or field checked U Unknown elev. source G Map elev., brn or not field checked V On vertical angle bench mark H Near vertical angle bench mark W Map elev., blue I Other special source X On or near boundary marker K Photogrammetry by other than USGS NMD Y Altimetry, poor control N Near USGS or NGS level-line bench mark Z Special source M On non-USGS level-line bench mark ____________________________________________________________________ Approximate Elevation gravity Elevation code--Second digit accuracy effect (ft) (mGal) ____________________________________________________________________ 1 On bench mark 0.2 0.01 2 Near bench mark 0.3 0.02 3 Transit or good alidade survey 1.0 0.06 4 VABM or black map elev. 2.0 0.12 5 Blk elev. on old map or good PG 4.0 0.24 6 Brn elev. or good PG on 20-ft contour map 10 0.6 7 Brn elev. on 80-ft contour map or good alt 20 1.2 8 Contour interpolation on 80-ft contour map 40 2.4 9 Contour interp. on 200-ft contour map, poor alt 80 4.8 ____________________________________________________________________ Aprox. Latitude code--Third digit Latitude Distance gravity (based at lat 37) accuracy accuracy effect (min) (ft) (mGal) ____________________________________________________________________ 1 Triangulation or special survey data 0.007 42 0.01 2 Location known to 0.04 in on 1:24,000 map 0.014 84 0.02 3 0.10 in, 1:24,000 or 0.04 in, 1:62,500 map 0.035 210 0.05 4 0.21 in, 1:24,000 or 0.08 in, 1:62,500 map 0.07 420 0.1 5 0.42 in, 1:24,000 or 0.16 in, 1:62,500 map 0.14 840 0.2 6 0.40 in, 1:62,500 or 0.1 in, 1:250,000 map 0.35 2,100 0.5 7 0.80 in, 1:62,500 or 0.2 in, 1:250,000 map 0.7 4,200 1.0 8 1.60 in, 1:62,500 or 0.4 in, 1:250,000 map 1.4 8,400 2.0 9 4.00 in, 1:62,500 or 1.0 in, 1:250,000 map 3.5 21,000 5.0 ____________________________________________________________________ Approximate Observed gravity code--Fourth digit gravity effect (mGal) ____________________________________________________________________ 1 Local survey with special gravity meter 0.01 2 Multiple observations with LaCoste and Romberg meter 0.02 3 Average LaCoste and Romberg or multiple Worden meter 0.05 4 LaCoste and Romberg observation with small vibrations or average Worden meter 0.1 5 Data from loop with closure error this large 0.2 6 Data from loop with closure error this large 0.5 7 Data from loop with closure error this large 1 8 Data from loop with closure error this large 2 9 Data from loop with closure error this large 4 ____________________________________________________________________